1. Field of the Invention
This invention relates generally to a laminated sheet and a bonded laminated sheet, and applications thereof, and more specifically to a laminated sheet suitable to be bonded by high-frequency welding, comprising a vinylidene fluoride resin layer, a bonding layer consisting of a specific resin composition, and a base-material layer; or a laminated sheet prepared by laminating a base-cloth layer with the base-material layer; and a method of bonding these sheets together by high-frequency welding; and applications of such laminated sheets to tent fabrics.
2. Description of the Background Art
Tents and tarpaulins are used for a wide variety of applications; tarpaulins being used as protective coverings in various applications, such as ships, trucks, railroad freight cars, civil engineering and construction, warehouses, and agricultural uses; and tents being used in a wide range of applications, such as shop-front shades, arcade tents, portable tents, structural tents. Various materials and structures best suited to specific applications are being adopted for tents and tarpaulins.
Tents and tarpaulins include those manufactured by bonding a top-surface layer sheet (overlay sheet) to a base cloth are publicly known. As the base cloth, canvas or other waterproof cloth made of flexible synthetic fiber, such as polyester, polyamide, vinylon, etc., or natural fiber, such as cotton, flax, etc. is most commonly used. A number of waterproofing methods are available, depending on varied waterproofing requirements ranging from applications requiring least waterproofing to those requiring perfect waterproofing. The water-repellency method in which cloth is treated with a water repellent, the dipping method in which cloth is immersed in a resin solution, the coating method in which cloth is coated with a paste resin, and the topping or laminating method in which a film is bonded to a base cloth are selectively used to meet such varied waterproofing applications. For portable tents requiring lightweight, for example, a vinylon cloth treated with the water repellency method, or a polyamide cloth treated with the coating method which requires only a small amount of resin is most commonly used.
The overlay sheet laminated to the surface of the base-cloth layer is required to have flexibility and weather resistance. As the overlay sheet, used is a three-layer laminated sheet in which a soft, vinyl chloride resin base-material layer is bonded to a vinylidene fluoride resin layer having good weather resistance as the top surface layer with a bonding layer interposed therebetween. As the bonding layer in this three-layer laminated sheet, poly-methyl methacrylate (PMMA) resin having good bonding properties has heretofore been used. The PMMA used in such a laminated sheet as the bonding layer, however, tends to be whitened when folded or bent during installation or sewing process, resulting in deteriorated external appearance of tarpaulins or tents due to white streaks in the bonding layer seen through a transparent vinylidene fluoride resin layer as the top-surface layer. To improve this drawback, a copolymer resin having good Flexibility of methyl methacrylate (MMA) and acrylic ester, such as butyl acrylate, has come to be used as the bonding layer.
When manufacturing tents, tarpaulins and other structures for various applications using laminated sheets consisting of an overlay sheet and a base cloth, as described above, a plurality of sheets cut to an appropriate length are usually used in the interests of workability. That is, the edges of these cut-length sheets are lapped and high-frequency welded to form a tent. In structures thus manufactured, however, the bonding strength of the joints often poses a problem, in addition to the above-mentioned whitening of the bonding layer. Bonding strength at relatively high temperatures is required of such structures, particularly when they are used in an environment where temperature rises due to direct sunshine. When two laminated sheets are bonded together by lapping the edges thereof, bonding strength depends on the bonding strength of the bonding layer in the underlying laminated sheet located in the vicinity of the lapped surfaces. When a base-cloth layer is used in each of the laminated sheets, the bonding strength of the bonding layer is particularly important, when considering the effects of the tensile strength of the base-cloth layer which is higher than the bonding strength of the bonding layer of the overlay sheet. Consequently, improving the bonding strength of the bonding layer of the overlay sheet, particularly the bonding strength of the bonding layer as the intermediate layer in the high-frequency welded overlay sheet, become a great concern.
When a copolymer resin of MMA and acrylic ester, such as butyl acrylate, is used as the bonding-layer resin, however, flexibility is improved and accordingly the whitening of the bonding layer due to folding can also be prevented, but the bonding strength of the bonding layer to the vinyl chloride resin layer by high-frequency welding becomes lower compared with the bonding layer of PMMA resin, resulting in lowered bonding strength when the laminated sheets are bonded together.
Japanese Published Unexamined Patent Application Hei3 (1991)-181581 discloses a technology in which a composition consisting of certain ratios of PMMA and vinylidene fluoride resin is used as an adhesive to hot-glue vinylidene fluoride resin to a rigid vinyl chloride resin molding. In the upper right column on Page 3 of the aforementioned publication, it is stated that a copolymer of acrylic ester and MMA, in place of PMMA, is also in the scope of analogical inference. The copolymer resin used in the present invention, however, is not specifically used in the aforementioned publication. The high-frequency welding process used in the present invention also is not adopted in the aforementioned publication, as will be described later.